The present invention proceeds from an arrangement having the features indicated in the preamble of Claim 1. An arrangement of this kind is known from DE 195 18 806 A1, where it is disclosed to monitor the pressure in the tires of a vehicle by means of pressure sensors which latter are arranged, together with a battery-driven electronic measuring and control circuit, a transmitter and a sending antenna, in the valve, on the valve or as one module with the valve of the wheel. This unit, being provided on the respective wheel, will be referred to hereafter also as wheel-mounted electronic module. The electronic tire-pressure signals supplied by the pressure sensor in the respective wheel-mounted electronic module are supplemented by the electronic measuring and control circuit by the respective identification signal, are then transformed to digital radio signals and transmitted by radio to a central receiving, evaluation and control unit, which latter is connected to receiving antennas for this purpose.
Contained in the radio signals as constituents in digital form are a preamble, the identification signal, a tire-pressure signal and a postamble. The radio signals are radio-frequency signals. In Germany, radio-frequency transmission is effected in the 433 MHz range, known as ISM band, in some other countries in the 315 MHz range or the 868 MHz range. The radio signals sent out by the wheel-mounted electronic module are received by the receiving antennas provided for this purpose.
When a separate receiving antenna is allocated to each wheel of the vehicle, it is preferably located in the neighbourhood of the respective wheel, in particular on the wall of the wheel house of the vehicle chassis. However, there is also the possibility to provide a common first antenna for the front wheels and a common second antenna for the rear wheels, which antennas are then located preferably approximately midway between the wheels. The receiving antennas transmit the RF signals via special lines, suited for transmitting RF signals, to the central receiving, evaluation and control unit where a RF receiver, provided with a separate receiving channel for each antenna, amplifies and demodulates the signal arriving at the input of the respective receiving channel. After demodulation, the LF signal then present is decoded and evaluated in order to provide the driver via a control module of the device with a warning, if necessary, or some other information regarding the tire pressure.
Since the wheel-mounted electronic module is battery-driven and the battery is to have a service life of typically seven years, the wheel-mounted electronic module must operate with the least possible current consumption. Its radio signals are, consequently, very weak. Due to the low signal level on the electric lines between the receiving antennas and the central receiving, evaluation and control unit, costly measures are required for the electric lines to protect the signals transmitted via those lines from external interference. In a practical example of a tire-pressure monitoring system according to DE 195 18 806 A1, the electric lines are twisted-pair cables that have been shielded in a costly way.
The safety from interference achievable in this way is only imperfect and connected with considerable cost.
Now, it is the object of the present invention to indicate an approach for making the transmission of the signals more immune to noise, and this in a low-cost way, without however reducing the service life of the battery in the wheel-mounted electronic module.
This object is achieved by a method having the features defined in Claim 1. Advantageous further developments of the invention are the subject-matter of the sub-claims.
According to the invention, the RF receiver with demodulator is no longer arranged at the central evaluation and control unit; instead, there is located at each receiving antenna a RF receiver, now only of the single-channel kind, with a demodulator so that the electronic lines have to transmit LF signals only instead of RF signals. For transmitting LF signals, no expensive shielded cables are required; rather, unshielded electric lines can be used that are available at much lower cost. An additional advantage lies in the fact that the LF signals formed already at the antenna, as a result of the demodulation step, are much less sensitive to interference than the RF signals transmitted via twisted-pair cables according to the state of the prior art.
Preferably, each individual receiving antenna is combined in one module with the allocated single-channel RF receiver with demodulator. This allows the shortest possible line connection between the receiving antenna and the single-channel RF receiver, and is especially favourable where low interference susceptibility is to be achieved.
The single-channel RF receiver must be supplied with voltage from the electric wiring of the vehicle. The voltage supply may be effected via a separate line connecting the electric wiring of the vehicle with the respective RF receiver. Preferably, however, the voltage supply is not effected via separate lines, but rather via the lines anyway provided for transmission of the LF signals from the central evaluation and control unit which in its turn is supplied from the electric wiring of the vehicle. The supply voltage being a direct voltage, the LF signal to be transmitted can be impressed by superimposition.
From DE 196 08 478 A1 it has been known that it is possible, by evaluation of the intensities (received amplitudes) of the radio signals received by the receiving antennas, to determine by means of the central evaluation and control unit the exact location at the vehicle of that wheel whose wheel-mounted electronic module sends out a signal with an individual identifier. If automatic determination of the wheel position by means of the arrangement according to the invention is to be rendered possible, then the central evaluation and control unit must be provided, additionally to the signal which is transmitted to it by radio and then demodulated, with some information regarding the intensity of the signal received by the respective receiving antenna, for example an information regarding the signal field strength received. This information is also described by the abbreviation RSSI in technical terminology (Received Signal Strength Indicator=measure of the received field strength). This can be achieved in different ways:
An analog signal having a strength proportional to the RSSI level of the signal transmitted by radio is produced in the single-channel RF receiver. For generating the analog signal, the signal received is fanned out, and one branch is integrated by a capacitor. The voltage building up at the capacitor then provides a measure of the RSSI level received.
The analog signal is transmitted to the central evaluation and control unit via a separate line as load-independent direct current. Preferably, the analog signal is transmitted via the same electric line by which the demodulated LF signal is transmitted. If in addition the voltage supply of the single-channel RF receiver is to be effected via the same line then the RSSI information cannot be transmitted as direct voltage, the latter being already used as supply voltage from the evaluation and control unit. Rather, the RSSI information may be transmitted in this case as current signal via the jointly used single line from the receiver/demodulator to the central evaluation and control unit where the LF signals and the RSSI signal can be separately evaluated.
On the other hand, the single-channel RF receiver can be suitably designed to locally evaluate the RSSI level of the signals received by radio, to convert the RSSI level to a digital signal and to transmit the latter together with the received digital signal, that has been received and then modulated, to the central evaluation and control unit. Transmission of the digital RSSI signal can be effected either before or after the received demodulated digital signal.
A further advantage of the invention lies in the fact that the assembly formed by the single-channel RF receiver and the demodulator can be suitably designed to generate at one interface the data protocol of a bus system installed in the motor vehicle. Linking the components consisting of receiving antennas, RF receivers and demodulators with the central evaluation and control unit can be effected through the existing bus system. By combining the respective receiving antenna according to the invention with an RF receiver and demodulator, the respective receiving antenna becomes an intelligent RF sensor with bus capability. The central evaluation and control unit can receive its tire-pressure information directly via a bus interface already available and can likewise output its output signals (indicator, diagnosis and control signals) via a bus interface. And the voltage supply of the RF receiver can also be effected via the bus system. A further connection, separate from the bus system, between the receiving antennas and the central evaluation and control unit is rendered superfluous by this development of the invention.